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S3 Puzzling leak from Master Brake Cylinder reservoir


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I have done a lot of work possibly to get back to stage 1.

The forward reservoir connection of my dual mbc on my 1978 109 was leaking so I replaced it, only then to find all slave cylinders needed replacing as bleed nipples shot.

Have eventually finished all round. Topped up the brake fluid. Rear brakes seem to have bled fine. Front ones seemed not to respond, then noticed pictured leak from front connection again. Talking with v helpful staff at onestopgearshop.com they say they are happy to exchange but do not understand why it would leak there as it should not be under pressure.

Have I had unhappy coincidence or is there something that might be astray within the system beyond the cylinder?

All help welcome - pretty new to this so apologies if any of this appears nonsensical or I gave missed any critical info...

Thank you


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The front brakes are a known person I'm not that keen on to bleed due to the bleed screw ports being on the horizontal center line creating air pockets above the bleed screws.

Remove the front drums and shoes and clamp the pistons fully back into the cylinders, then bleed them out. Refit the shoes and drums, adjust and you should have brakes.

Regarding the master cylinder, i would say that the res to cylinder seal is missing/defective in that port.

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I'd agree that the master cylinder's port must be defective, either because of scoring or inaccurate machining, a split o-ring, or even the rust spreading past the o-ring.

As for bleeding the front brakes, they are difficult for two reasons: a) the ports are on the sides of the slave cylinders, not the top, trapping air, and b) the PDWA valve on the chassis is also an air trap. The best way of bleeding the brakes on a dual circuit system, therefore, is to bleed from the wheels up to the reservoir. This means that the fluid enters the PDWA valve from the bottom, the air rising out of the top easily. It also means that the pressure bleed leaks fluid out past the nipple threads rather than air back in on pedal bleeding. As for twin leading shoe systems on the front and their port positions trapping air, most of the problem can be eliminated by bleeding with the pistons clamped full retracted as Phil said, but can only be eliminated entirely by vacuum bleeding or by removing the wheel hubs from the axle and the brake units, complete with back-plate, front the stub axle to lay horizontally to reorientate the cylinders so that the ports are all at the top of the cylinders. It's a nuisance, but look on it as a opportunity to service the wheel bearings and hub seals!

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What bits have you changed? If the seals have gone in the master cylinder it is perfectly possible for it to pressurise the reservoir when you pump the pedal. This would also make the brakes not work on that circuit of course. Never had any of the problems mentioned when bleeding TLS brakes by the way, just used to wind the adjusters up tight first and use the two man "up-down" method.

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